Surfaces that remain dry under a flux of steam are desired for many applications, such as the blades of steam turbines and the surfaces of heat exchangers. Different techniques are employed to create water-repellent surfaces, commonly referred to as superhydrophobic surfaces. They are defined as surfaces on which water forms a contact angle of greater than 150° and are formed by a suitable combination of surface chemistry and roughness. However, the ability of a surface to remain dry by letting water droplets roll off depends on the contact angle hysteresis (CAH) defined as the difference between advancing and receding water contact angles on a surface. The major factor influencing the CAH on a superhydrophobic surface is the superhydrophobic state of the water droplets. A droplet of water on such a surface may be in a wetting state (Wenzel state) with water completely filling the roughness asperities on the surface or a nonwetting state (Cassie-Baxter state) with the water droplet supported on the air-solid composite surface. In the Cassie-Baxter state, water droplets have lower CAH and easily roll off the surface as compared to those in the Wenzel state.
Compared to the water repellency of a superhydrophobic surface where a water droplet is deposited on the surface, in the case of the condensation of water vapor on such a surface the formation of a water droplet follows a nucleation and growth mechanism where nucleation may be initiated within the pores. This would result in the formation of water droplets in the Wenzel state. Therefore, a superhydrophobic surface that has a low contact angle hysteresis with liquid water may not show similar behavior in the case of steam condensation. For the surface to remain dry under condensation, it is required that the water droplets undergo a transition from the Wenzel to the Cassie-Baxter regime. Depending on if this transition is favorable thermodynamically, a typical superhydrophobic surface may or may not act as an anticondensation surface. Few approaches proposed and tested in the past to create surfaces that can undergo such a transition include the design of surfaces with differential intrinsic wettability in the form of either a continuous gradient or patches of hydrophilic-hydrophobic regions on the surface. A similar vertical surface energy gradient was proposed to explain the ability of a lotus leaf to remain dry under dew formation. The systematic design of roughness features on the surface such that the nanoscale roughness is present as either part of the structural hierarchy or the surface being nanoporous by itself has also been shown as a design approach to making a surface remain dry under condensation. However, the exact mechanism behind the wetting behavior of a structured surface under condensation is still not understood completely. All of the condensation studies reported pertain to the low temperature and pressure of the condensing water vapor, typically carried out in the range of 0-5° C. In the case of steam condensation, the mechanism of droplet formation is similar to low-temperature condensation. However, the high-temperature, high-pressure environment of condensing steam to which the surface is exposed may exceed the mechanical and hydrostatic stability limits of the nonwetting state of the surface. Thus, the stability limitations make it more difficult to design a steamphobic surface even though control over the surface energy and geometry is easily achieved with a wide range of materials.
This work creates unique coatings for stainless steel that remain dry under prolonged exposure to steam and maintain their superhydrophobicity. The coatings are formed using a meshlike carbon nanotube (CNT) structure. The structure provides the desired surface roughness and porosity. The chemistry of CNT structures is modified using plasma-enhanced chemical vapor deposition (PECVD), more commonly known as the plasma polymerization process.